As it is known in the art, integrated circuit boards, or modules, generally comprise a number of integrated circuit packages, or dies, which are coupled via pads onto a printed circuit board (PCB) using solder. One type of integrated circuit package is a Ball Grid Array (BGA). The BGA package may be, for example, a memory chip with solder balls on the underside for mounting. The BGA package has its Input/Output (I/O) pins spread across the whole of the underside surface area in a checkerboard pattern, rather than just at the periphery. The use of BGAs allows die package size to be reduced because there is more surface area for attachment. Smaller packaging allows more components to be mounted on a module, making greater densities available.
BGAs are reflow soldered to PCBs using a mass reflow process. During the reflow process the PCBs are passed into a furnace on a conveyor system, and hot air is forced into the processing chamber through a high number of holes or nozzles and onto the PCBs passing on the conveyor system. The air then heats the PCBs to melt the solder paste alloy and form solder joints between the BGAs and the PCBs.
After the PCB is populated, it may undergo tests and be released to the field. At any time it may occur that one or more of the integrated circuits mounted on the PCB fails and needs to be replaced. When one or more of the integrated circuits fails, the module needs to be reworked. During the rework process the failing integrated circuits are removed from the module and replaced with operative integrated circuits. It is important that the integrity and reliability of the solder joints between the replacement IC and the PCB are maintained.
The standard approach for reworking a BGA is to place the replacement BGA into a placement nozzle and hold it into position with a vacuum cup, aligning the balls of the BGA with the solder footprint. The BGA is then dropped into position on the PCB. The placement nozzle is then partially lifted, although it may still shroud the component. As shown in FIG. 1, a flow of heated forced convention gas is applied through nozzle 14 onto the body of the component 16. The flow of air drives the heat through the component body to the solder ball array on the underside of the BGA 16, thereby melting the solder and forming a solder joint on the PCB 12.
There are several problems with the existing rework approach. One problem arises due to the density of placement of ICs on the PCB. In order to maximize the use of the PCB space the ICs are generally closely spaced. As a result, residual heat from the stream of hot air that is directed at the reworked IC may undesirably affect the solder joints of neighboring ICs, for example causing the solder connections to become liquidus, and dislodging the components from their appropriate positions on the PCB. In order to preclude this occurrence, it is often necessary to place ceramic covers, such as cover 15 over neighboring ICs.
Another disadvantage of this approach results from the natural temperature drop of the air flow on its path from the nozzle to the underside of the BGA. The initial temperature of the air should be high enough to ensure that, despite the temperature drop, the desired temperature to melt the solder between the BGA and the PCB is attained. In addition, the initial temperature may have to be adjusted even higher to accommodate the dissipation of the air flow as it is directed at the target component. For example, a starting temperature of 320 to 340 degrees or greater may be required to heat the solder interconnects to a desired 200 to 210 degrees. A problem exists in that the component 16 may not be warranted to operate effectively under these temperature conditions. For example, a typical integrated circuit may have an operating specification threshold of only 240 degrees. The additional heat can also physically distort the package. This problem is exacerbated when solder paste made from material having a higher melting temperature is used. As the trend away from lead based solder continues, this issue will rise in importance.
Another disadvantage of the current rework practice is that it makes it difficult to obtain the thermal profile of the solder set point. Very specific time and temperature values are required in order to properly establish a reliable solder interconnect. Measuring the temperatures and then adjusting the air flow, temperature and time to achieve the specific required time and temperatures facilitate thermal profiling. Because BGAs come in a great deal of sizes, the way that the air flow characteristics affect the solder set points of the BGAs are unique to each. Using an open nozzle design such as that shown in FIG. 1 forces the process of identifying the thermal profile of a given BGA to be trial and error, often taking many hours or days to establish, and can vary with different ambient conditions.
The current rework process also is also challenged by the increasing weight and densities of the BGAs and other integrated circuit components. Component weights are increasing as a result of increased power dissipation. As the weight increases it is important to increase the precision of the temperature application to the component during rework. If the temperature application is imprecise, solder shorting and collapse of the solder ball during rework may occur. Solder shorts are one of the primary defects in BGA rework. The defect is created when two connections are allowed to join during the soldering process. When a single shorting occurs, the component must be disposed. If shorting should occur multiple times, the entire board is disposed, often at significant expense.
A further problem of the existing rework process is the difficulty associated with proper alignment of the replacement BGA on the PCB. Usually a complicated vision alignment system including microscopes and mirrors is used to properly position the BGA on the PCB. The system is operator dependent, requires a high level of expertise and thus is often prone to error and misplacement. Misalignment of components is a significant defect and a common byproduct of the current rework process.